The present invention relates generally to integrated electronic circuits and semiconductor memory devices, and more particularly, to using isolation of active regions in MOS integrated circuits to radiation harden semiconductor static random access memory cells.
In today""s metal-oxide semiconductor (xe2x80x9cMOSxe2x80x9d) integrated circuit design, active regions on a chip are isolated from each other by a thick layer of thermally grown oxide, known as a field oxide, overlying doped channel-stop regions. This method of isolation has a number of disadvantages that become increasingly apparent with higher component density on the chip. The field oxide grows in areas not covered by a nitride mask layer used to define the active regions during processing. The use of a nitride mask forms a characteristic xe2x80x9cbird""s beakxe2x80x9d shape in the oxide that consumes what would otherwise be usable active area while insignificantly contributing to the isolation function of the field oxide. The channel-stop dopants may also diffuse into the active area upon thermal growth of the field oxide, causing a narrow channel effect and increasing required threshold voltages for components constructed in the active regions. Further, mobile ions arising from electrical stress can cause shifts in the field threshold voltage, resulting in inconsistent performance of active circuit components.
In an isolation structure known as the xe2x80x9csea-of-gates,xe2x80x9d every other one of transistors has a grounded gate, thereby isolating those transistors without grounded gates on each of two opposite sides, while the traditional field oxide and channel stops are used to complete the isolation. However, all of the transistors in xe2x80x9csea-of-gatexe2x80x9d design are of identical construction, and those with grounded gates are not therefore expressly designed or tailored for the isolation function they serve.
With increasing component density in today""s integrated circuit design, it is highly desirable to provide effective isolation of active regions on a chip while minimizing the consumption of otherwise usable active area unnecessary to the isolation function.
In addition to the issues of integration density, conventional oxide isolation regions may be limited in their applicability in particular environments. For example, semiconductor memory devices used in outer space, such as in a satellite, are subjected to severe environmental conditions that may compromise the integrity of the stored data, or cause the memory devices to fail. In many cases, the memory devices are part of a larger embedded system, where the memory device is just one of many devices sharing the same die. The integrity of the memory devices used in outer space applications is critical because the information stored by the memory devices may be related to critical functions, such as guidance, positioning, and transmitting and receiving data from a ground base station. Furthermore, semiconductor memory devices for use in space applications should remain functional for the lifetime of the satellite, which may be as long as several years. Contrast this with applications where the memory devices are also subjected to harsh operating conditions, such as guidance systems in missiles, but only for a relatively short time period.
One cause of errors in semiconductor memory devices that are used in outer space applications is due to high-energy particles impinging on the memory device. There are several forms of high energy particles in outer space. For example, there are alpha particles and gamma rays, to name a couple. These high-energy particles strike the semiconductor material on which the memory devices are formed with enough energy to cause the generation of electron-hole pairs. The resulting charge carriers are often trapped in the various oxide layers of the memory devices. In the case of metal oxide semiconductor (xe2x80x9cMOSxe2x80x9d) transistors, charges trapped in the gate oxide will shift the threshold voltage, Vt, of the transistor. As a result, leakage currents of the transistors, and consequently, of the memory devices may increase. Where the transistor is used as a transfer gate for a conventional memory cell, the increased leakage current may compromise the integrity of the data stored by the data storage node, such as a capacitor, by allowing the charge representing the data to dissipate.
The frequency or number of charges trapped in an oxide layer is proportional to the thickness of the oxide layer. Consequently, oxides having a greater thickness will, on the average, have a greater number of trapped charges. In the case where the oxide is relatively thick, for example, approximately 4000-5000 xc3x85, charge trapped in the oxide will result in a much more dramatic shift in the leakage current characteristics than for an active transistor having a relatively thin gate oxide. Such an application of thick oxide is for isolating active transistor areas in which memory cells may be formed, for example, regions of local oxidation of silicon (xe2x80x9cLOCOSxe2x80x9d). The accumulating charge trapped in the LOCOS region may become great enough to cause a conduction channel to form below the LOCOS region, and consequently allow current to leak between neighboring active transistor regions.
As mentioned previously, transistors in the active region of a memory device, which typically have gate oxides that are much thinner than the oxides of the LOCOS region, will have shifting Vts as a result of the trapped charges in the gate oxide. A method that has been used to accommodate the shifting Vts is to raise the Vts of the active transistors so that the relative changes in the Vts due to the trapped charges are minor. Thus, the effect that trapped charges will have on the overall performance of the memory devices is minimized. However, the aforementioned technique is not as effective when applied to LOCOS isolation regions. Although the Vt of the incidental transistor of the LOCOS isolation regions may be adjusted so that the Vt shift due to charge trapped in the thick oxide is relatively small, the resulting isolation region will have an unacceptably low breakdown voltage.
Another technique used to accommodate charge trapped in a thick LOCOS isolation region is to create a two-layer oxide isolation structure. A relatively thin layer of oxide of approximately 500 xc3x85 is grown in the isolation region, followed by the formation of a relatively thick layer of phosphorous doped oxide of approximately 4000 xc3x85 over the relatively thin layer of oxide. The phosphorous doped oxide is subsequently densified by baking. The resulting structure minimizes the effect trapped positive charges have on the leakage characteristics of the isolation region by neutralizing the trapped positive charges with the abundance of electrons of the relatively thick phosphorous doped oxide. However, this approach is limited by the doping concentration of the relatively thick oxide layer. That is, the greater the level of impurities, the greater the diffusion of the impurities from the thick oxide into the adjacent layers.
Another approach taken in providing semiconductor memory devices suitable for use in space applications is through a method of trial and error. Samples of potentially applicable semiconductor memory devices are taken from several different process lots and subjected to severe radiation conditions that simulate the environment of outer space. The sample devices are subsequently tested for functionality and reliability. Those memory devices from the process lots having samples surviving the testing are judged to be acceptable for use in applications in outer space. However, this approach merely screens existing memory devices for those which may be sufficient for use in outer space applications, and consequently, the quantity of satisfactory memory devices produced by this method is subject to the unpredictable nature of semiconductor processing.
Therefore, there is a need for a semiconductor memory device having reliable data retention over a sufficiently long period of time for use in applications subjected to severe radiation environments, such as outer space.
According to the present invention, static random access memory (xe2x80x9cSRAMxe2x80x9d) cells are formed having isolation gate structures constructed between or around active areas of a semiconductor substrate by forming an insulating layer on the surface of the substrate, forming a polycrystalline silicon layer on the insulating layer, and selectively etching away portions of the deposited layers to expose the active areas of the semiconductor substrate for further processing. Each of the memory cells has data storage nodes that are selectively coupled through transfer gates to a respective sense node. The data storage nodes are electrically isolated from each other, and the data storage nodes of adjacent memory cells by an active gate isolation structure that is formed in between the nodes and coupled to a voltage potential. The active gate isolation structure prevents a conductive channel extending between the data storage nodes of the memory cells from forming. The data storage nodes of the memory cells may be further isolated from one another by an oxide isolation structure formed in series with the active gate isolation structure and in between the data storage nodes.